It's done primarily for noise reduction, but easier maintenance access, more room for cargo, pax and/or kerosene as well as aerodynamic considerations are also part of the equation. Engines podded below and in front of the leading edge make excellent places for vortex generators to help energize the boundary layer on the wing to help handling at high AOA and improve stall characteristics.
The primary drawback is that an engine fitted outside of the fuse or wing produces parasite drag.
Last edited by A-FLOOR; 12th July 2006 at 17:00.